Method of purifying ε-caprolactam

ABSTRACT

Crude caprolactam is purified by reaction with a complex hydride of boron or of aluminum.

DESCRIPTION

The present invention relates to a process for purifying ε-caprolactam.

ε-Caprolactam is an important starting material for the production ofpolyamides (nylon 6). There are various ways of producing itindustrially. The most popular option is by Beckmann rearrangement ofcyclohexanone oxime (K. Weissermel, H. J. Arpe, Industrielle OrganischeChemie, 4th edition, pp. 272). Alternatively, cyclohexanecarboxylic acidis produced from toluene via benzoic acid and rearranged withnitrosylsulfuric acid to the ε-caprolactam. Other processes are based onthe cyclization of ω-aminocaproic acid derivatives, for example6-aminocaproic esters (EP-A 376 123) or 6-aminocapronitrile (EP 659741), in the presence of suitable, typically acidic, catalysts to formε-caprolactam.

All ε-caprolactam processes give rise to by-products, the nature andquantity of which depend on the principle of the process, on the qualityof the starting materials and also on the process parameters. On theother hand, the ε-caprolactam has to meet high purity requirements,especially in fibermaking. For this reason, each manufacturing processrequires its own optimized purification process. The variouspurification processes are cited for example in Process EconomicsProgram Report No. 41 B, Caprolactam and Nylon 6, March 1988, pp. 69.

These purification processes are generally combinations of extraction,distillation and/or crystallization processes. Highly contaminatedcaprolactam fractions, for example caprolactam purification residues,are frequently subjected to a catalytic hydrogenation. Removal of thecatalyst is generally followed by a distillative workup or the returninto the purification cycle. In the case of a catalytic suspensionhydrogenation of the crude ε-caprolactam using Raney nickel (EP-A-138241, JP-A-60-21145) the removal of the catalyst presents problems. Inthe case of a hydrogenation of the crude ε-caprolactam over fixed-bedcatalysts (DE-A-1004616, DD-A-75083), decreasing activity or poisoningof the catalysts is likely over time.

It is an object of the present invention to provide a low-costuniversally deployable purification process for ε-caprolactam.

We have found that this object is achieved, surprisingly, by using acomplex hydride of aluminum or of boron.

The present invention accordingly provides a process for purifyingε-caprolactam, which comprises reacting crude ε-caprolactam with acomplex hydride of aluminum or of boron.

Suitable complex hydrides of aluminum or of boron for the process ofthis invention are in particular sodium borohydride, lithiumborohydride, potassium borohydride, calcium borohydride, sodiumcyanoborohydride, sodium methoxyethoxyaluminum hydride, lithiumtri-t-butoxyaluminum hydride.

The amount of hydride hydrogen used naturally depends on theconcentration of the impurities in the ε-caprolactam. It is chosen sothat an excess of hydride hydrogen is present, based on the impuritiesto be reduced. Preference is given to an excess of from 1.5 to fivetimes, based on the stoichiometrically required hydride quantity.

It has been found that, to obtain an adequate reaction rate in theε-caprolactam purification process of this invention, from 10 to 50% byweight of water, based on crude ε-caprolactam, has to be added to thereaction mixture when a borohydride is used.

In a preferred embodiment, the reaction is carried out in the presenceof from 0.5 to 5 mol %, in particular from 1 to 4 mol % of NaBH₄ and offrom 10 to 50% by weight of water, based on crude ε-caprolactam. Thesodium borohydride can be used in solid form or in the form of acommercially available aqueous solution.

The reaction is preferably carried out within the range from 10 to 150°C., in particular within the range from 20 to 100 ° C. The reaction timeis within the range from 0.5 h to 200 h, preferably within the rangefrom 1 h to 100 h.

After the reaction has ended, water and the reduced impurities aredistilled off under a slightly reduced pressure. It is also possible todistil off water and impurities continuously even during the reaction byapplying a slight reduced pressure. Thereafter the reaction mixture issubjected to a conventional distillation under reduced pressure (0.5-8mm Hg). Pure ε-caprolactam is obtained as distillate with a UV number<10(see below).

The process of this invention is suitable for purifying ε-caprolactamproduced by any of the conventional manufacturing processes. Thisprocess is advantageously suitable for purifying ε-caprolactam producedby cyclization of ω-aminocaproic acid derivatives, for exampleω-aminocaproic acid, ω-aminocaproamide, ω-aminocaproic ester andω-aminocapronitrile.

EXAMPLES

Purification of a Caprolactam Produced by Cyclization ofω-aminocapronitrile

The purity of the ε-caprolactam is determined in terms of the UV number.The UV number is defined as the sum of all absorbances of a 50% strengthby weight aqueous caprolactam solution measured at intervals of 10 nm inthe wavelength range from 280 nm to 400 nm in a cell having a pathlengthof d=5 cm. The UV number of the purified ε-caprolactam should not exceed10.

Example 1

Preparation of ε-caprolactam in Accordance with DE-A 43 396 48

100 parts by weight of ω-aminocapronitrile were dissolved in 1000 partsby weight of ethanol and 30 parts by weight of water and passed over afixed bed of titania (anatase) at 220° C. with a residence time of 12min. The solvent was then distilled off. The crude caprolactam thusobtained was used in the following purification stage.

Purification of the Crude ε-caprolactam

70 parts by weight of crude caprolactam and 30 parts by weight of waterwere admixed with 1% by weight of sodium borohydride (based on crudeε-caprolactam) in a stirred vessel equipped with a still head. After 100h, the reaction mixture was subjected to fractional distillation.ε-Caprolactam passed over at 119° C. (2 mbar) with a UV number of 3.1.

Example 2

Crude caprolactam prepared by Beckmann rearrangement (see ProcessEconomics Program Report (SRI Report) No. 41 B, Caprolactam and Nylon 6,March 1988) was reacted as described in Example 1 in the presence of0.4% by weight NaBH₄. After 24 h, the reaction mixture was subjected tofractional distillation. ε-Caprolactam passed over at 125° C. (3 mbar)with a UV number of 3.5.

I claim:
 1. A process for purifying ε-caprolactam, which comprisesreacting ε-caprolactam with a complex hydride of aluminum or of boronacidic substances.
 2. A process as claimed in claim 1, wherein thecomplex hydride is selected from the group consisting of NaBH₄, LiBH₄,KBH₄, Ca(BH₄)₂, and NaBH₃ CN.
 3. A process as claimed in claim 1,wherein from 2 to 20 mol % of hydridic hydrogen is used, based on theε-caprolactam to be purified.
 4. A process as claimed in claim 1,wherein ε-caprolactam is reacted in the presence of from 0.5 to 5 mol %of NaBH₄ and from 10 to 50% by weight of water, based on theε-caprolactam to be purified.